The invention relates to improvements in paper machine dryer drums, and more particularly to an improved condensate removal system with a shoe capable of improved removal of condensate forming on the inner surface of the drum.
In a papermaking machine after the web is dewatered and pressed, it is passed to the dryer section which conventionally embodies a plurality of cast iron steam heated rotating dryer cylinders. Steam enters each of these dryers and condenses as it transfers heat to the dryer shell, and the dryer shell in turn transfers heat to the paper causing the moisture in the paper to evaporate. It is essential to producing a satisfactory paper sheet that the dryer drum transfer heat efficiently and uniformly to the sheet, and it is essential to satisfactory and economical operation that effective and efficient heat transfer occur from the steam in the drum to the shell. A large factor in controlling and attaining effective heat transfer is the removal of the condensate within the drum. This condensate may be ponding, cascading or rimming, and the state of the condensate depends upon a number of factors, but the siphon assembly which is provided to remove the condensate must be capable of satisfactory removal whether the condensate is ponding, cascading or rimming.
When the condensate is rimming, the heat must be transferred through it, and this is a thermal problem. The thicker the condensate layer, the higher the thermal resistance. When the condensate is not rimming, the problem becomes a mechanical one, and if the condensate layer becomes too large, it takes longer to speed up, and the drive loads increase and the drive torque becomes erratic which has a deleterious effect on the quality of the paper being manufactured. Therefore, it is important to minimize the amount of condensate in the dryer drum at all times.
It is, therefore, an important object of the present invention to provide an improved dryer drum and siphon condensate removal system which minimizes the amount of condensate in the drum at all times and is capable of handling the condensate whether it be ponding, cascading or rimming.
In general in present commercial dryers, the steam enters the dryer drum often through a backside journal, and the condensate is evacuated through a frontside journal along with some uncondensed steam which is termed blow-through. The components of the siphon assembly are the siphon shoe which is usually positioned adjacent the inner surace of the shell, the radial pipe through which the condensate flows after it is picked up by the shoe, the siphon elbow which turns the flow of condensate to a horizontal direction, the horizontal pipe, and the rotary steam fit. These items make up the dryer siphon assembly and are each important, but a salient feature of the instant invention is the design of the shoe which is capable of effective removal of the condensate and thorough more complete removal for the conservation of energy and the production of a better paper web.
The achieving of a structure which can operate with a thinner layer of condensate will reduce the insulating effect of the condensate. It has been found that the condensate depth decreases consistently with decreasing clearance between a siphon shoe to a point of approximately 0.040". Once that point has been reached, the condensate cannot be reduced in thickness by any further reduction in siphon clearance, and this feature is taken into consideration in connection with the design of the siphon shoe of the instant invention.
As stated above, the condensate may take different forms in either cascading, puddling or rimming, and it is well known as recited in White, R. E. "Residual Condensate, Condensate Behavior, and Siphoning in Paper Driers", TAPPI, Vol. 39, No. 4, April 1956, p. 228-233, that at lower dryer speeds, condensate which is still in the dryer drum will form a puddle in the bottom of the dryer. At slightly higher speeds, this condensate starts to climb the wall of the dryer shell and then cascade back into the puddle. An excessive amount of condensate cascading off the dryer wall will produce a large and erratic drive load. At higher speeds, the condensate forms a thin circumferential layer on the inside dryer surface, and this layer acts as a thermal insulator. Under these rimming conditions, an excessive amount of condensate in the dryer will produce an excessive thermal resistance. To avoid both of these operating conditions, the amount of condensate in the dryer must be kept at a minimum, and in accordance with the features of the present invention, the design of the siphon shoe greatly influences the value of this minimum. During nonrimming conditions, the siphon shoe will evacuate condensate only when it is submerged in the condensate puddle, that is, when the siphon shoe is in the region where it projects downwardly so as to be in the puddle. During the rest of the cycle, the siphon shoe is exposed only to the steam which is under pressure in the drum and hence only blow-through steam is being evacuated, rather than condensate.
In order to evacuate as much condensate as possible during the time the siphon shoe is submerged, conventional siphon shoes have heretofore been designed for nonrimming conditions and have a large spacing between the siphon shoe and the dryer shell, or a large opening in the bottom of the siphon shoe. With this construction and the use of a large opening, the blow-through flow rate of steam is excessive when the siphon shoe is not submerged. Further, the amount of condensate in the dryer is very large during high speed rimming operation.
In accordance with the features of the present invention, the foregoing problems are minimized by the design of the siphon shoe such that a relatively small spacing, optimally 0.040" is provided between the siphon shoe and the inner surface of the dryer shell. It has been discussed that the spacing has an effect on the thickness of the layer of rimming condensate by Calkins, D. L., "The Effects of Siphon Clearance on Dryer Performance", The Johnson Corporation, Three Rivers, MI., 1966. The small spacing in conventional designs, however, has a disadvantage of providing a very poor nonrimming performance.
A further feature of the invention substantially improves the condensate removal performance in that the profile of the edge of the siphon shoe which first contacts the condensate puddle is uniquely constructed. This edge slopes in an angular face toward the inner surface of the shell and has a large projected opening which is tapered down to a small siphon shoe to dryer shell spacing. The angle of the ram face which faces in the direction of rotation of the dryer should be in the range of 10.degree. to 45.degree. and of a height which is preferably in the range of 0.2" to 1.0". This leading edge acts as a channel which forces the condensate into the small opening and has a ram effect greatly enhancing the nonrimming performance of the siphon which is mounted very close to the dryer shell.
A further object of the invention is to provide an improved siphon shoe which extends axially along the shell for an optimum length and which is of improved design construction for receiving and removing condensate forming on the inner surface of the shell.
A further feature of the invention is to provide a siphon shoe with a center enlarged chamber portion.
A further object of the invention is to provide an improved siphon shoe of a design which is structually efficient to manufacture and which is capable of the improved removal of condensate from a steam dryer drum.